Just watched this : http://docs.macsysadmin.se/2017/video/Day3Session4.mp4
Basically APFS is fragmentation machine on steroid, every change in a file creates fragmentation, "thanks" to its core feature "copy-on-write" and even though SSD are much less impacted than HDDs they're also impacted.
Moreover, it much more CPU intensive than HFS+ and benchmarks proved it 20% slower and even worse.
Given Filemaker reliance on disk I/Os that gets me very concerned. I hope FMI mitigate this.
It's even more important because 10.4 may be APFS only. In that case FMS Mac Install would suffer a considerable speed penalty.
And it seems that that the speed gets worse and worse as the number of edits occurs. For filemaker files and caches that gets constantly changed that will be a nightmare